Troubleshooting IPv6 Automatic 6to4 Tunnel

In this lesson we’ll take a look how to troubleshoot an automatic IPv6 6to4 tunnel. Take a look at the following topology:

This scenario requires some more explanation. R1 and R3 each have a loopback0 interface with an IPv6 prefix on it. The FastEthernet interfaces of R1, R2 and R3 only have IPv4 addresses. The network engineer that designed this topology created an automatic 6to4 tunnel to establish connectivity between the two IPv6 networks. Of course this is not working so we’ll have some fixing to do.

The tunnel 0 interfaces are sourced from the FastEthernet 0/0 interfaces of R1 and R3. By sending a ping from R1 to R3 between the FastEthernet 0/0 interfaces I know that IPv4 routing is not the issue. Let’s take a closer look at the IPv6 addresses:

Taking a quick look at the interfaces tells us that the IPv6 addresses on the loopback 0 interfaces have been configured correctly and that the interfaces are up and running. You can also see the IPv6 addresses on the tunnel 0 interfaces. What do the 2002:A0A:A01::1 and 2002:A14:1403::3 addresses mean?

Keep in mind that the tunnel mode is automatic 6to4. The “automatic” part means that the tunnel destination IP is not configured statically but within the IPv6 address. 2002::/16 is the range that is reserved for tunnels. Let’s see if we can ping these tunnel IPv6 addresses:

We’ll remove the IPv6 address and configure another one that falls within the 2002:A14:1403::3 range.

Now we know that the IPv6 addresses on the tunnel 0 interfaces are correct. Next step is to check our routing to see if R1 and R3 know how to reach each other’s IPv6 addresses. Let’s check the routing tables:

There are two static routes here. The 2001:1::3/128 points to the loopback 0 interface of R3 but the next hop IPv6 address is 2002:A14:1403::3. This next hop is incorrect so we’ll have to change it. The static route for 2002::/16 to the tunnel 0 interface is fine. This prefix is reserved for tunneling and we need it because the router will do a recursive routing looking when it tries to reach 2001:1::3/128. Let’s fix it:

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